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Indoor Artificial Star and Null Test

Have you ever received a new prized telescope only to have
cloudy skies for what seems to be weeks at a time? We've all felt the desire to get out with our new scopes and
put it through its paces only to have miserable weather night after night. Often when it is clear the seeing makes
stars look like bloated balls and star testing is all but impossible. Well it's possible that you may already
have the materials necessary to star test your new scope indoors under
controlled conditions in perfect seeing with star images at infinity. It's also possible that you can with a
similar test but with the use of a knife-edge test the optics in your prized
scope with a very sensitive test that you may have thought only possible in an
optical shop with very expensive equipment.

The key to this sensitive test is the use of an artificial
star but with a twist. Ordinarily
we use our telescopes to bring parallel light from a point source to converge
to a perfect point at focus. If
however the reverse is done, that is to place a point source at the focus then
light will bounce off the primary mirror or pass through the lens and come out
being perfectly parallel provided the main optic is of high quality. The result is an artificial star is
produced at infinity for anyone literally looking into the objective but with
essentially perfect seeing. This
allows some level of optical evaluation to be conducted indoors under controlled
conditions. The key to being able
to do this requires that we already have another scope known to be of high
optical quality, or a control telescope, and the ability to make a very high
quality artificial star that can be inserted into your scope's focuser.

How to make a focuser
insertable artificial star

Most commonly available artificial stars such as the one
from EZ Telescopes alone are not good enough as the light source is too
large. If however some optical
elements such as an eyepiece or two are placed in front of the light source the
effective diameter can be significantly reduced.

In my case I used 2 inexpensive .965-inch eyepieces. One eyepiece had it's barrel removed
and the optics were taped right next to the hole opening of the artificial star
and the other was placed about 3 inches away on the other end of the
tele-extender. Duct tape was used
to hold everything in place to assemble a focuser insertable artificial
star. I found that the best
compromise between brightness and sensitivity is an artificial star that is about
3-5 arcseconds in angular diameter.
If the resulting star image is too large, try inserting a Barlow lens in
the optical path. It will further
reduce the size of the star in increase sensitivity. It's important to
make sure that the eyepieces used are
clean and free of dust or imperfections or blemishes will interfere with the
resulting test images.

Star test your scope
indoors at infinity

To star test a telescope indoors, the tester needs to
locate the exact position the focuser needs to be at to simulate infinity for
one of the two scopes being used in the test. To do this, take a casual look
through the other scope and focus at infinity at a distant object.
Then literally aim that scope right into the objective of the other
scope that has the artificial star placed into it and an out of focus star
should be seen once aimed properly.
It's then necessary to focus the scope with the artificial star in it so
that the image through the scope with the eyepiece is focused or nearly
focused. This assures that the
star is EFFECTIVELY at infinity.
It's best to somehow record the approximate position of the focuser so
that this procedure only needs to be done once. Then a normal star test can
be conducted just as outside but with no thermal issues. If the
diffraction patterns appear fuzzy or ill defined then the artificial star is
still too large and a different combination of lenses should be used. Either
smaller focal length eyepieces or a greater number of eyepieces should work.
The smaller the airy disk produced, the more sensitive the
test but at the cost of having a dimmer light source. It needs to be mentioned
here that it is essential that the control telescope needs to be of high optical
quality AND properly collimated
for this or the outgoing light may not be perfectly parallel.

Optics 101 and the null
test

For most telescope enthusiasts, this might be as far as
one would desire to go but it is possible to take the optical testing to the
next level. To explain further, I
need to digress to another type of optical test that very few amateurs actually
do on their scopes: the null test.
When an optician is making a parabaloidal telescope mirror one of the
steps is to make the glass a sphere.
At this point, the Foucault image shows that the glass has no correction
and looks essentially flat with no apparent hills or valleys. The image shows
a perfect null and deviations from this can easily be seen.
Then the optician goes on to add correction until the measurements show
it to have all the right deviations from sphere at the right places, which can
be very difficult to verify unless the tester is experienced.

Other testing methods can be set up in the optical shop so
as to show a perfect parabaloid as a perfect null, which is highly desirable
because any deviations from optical perfection should ideally show up clearly
in a 3D shadowgram. But
large aperture precision optical flats and/or precision null lenses are needed
for this and the costs and difficulties in setup are very high. So most opticians
rely on their ability to interpret Foucault images they see before deciding that
a mirror is done and
for many inexperienced glass pushers they may not have a very good optic but
will have to unfortunately wait until they see the questionable images before
knowing for sure what they have.
If a knife-edge is placed in the focuser of a completed scope while
focused on a bright star then, because the point source is at infinity, a
perfect parabaloidal mirror will show that perfect null that is so much more
easily interpreted than a with a Foucault test in the optical shop. To set up
this test, an accurate
knife-edge must first be placed in the focuser.

How to make a focuser
insertable knife-edge

It is important to have an accurate and very straight
knife-edge that cuts right through the halfway point of the focuser. To do this,
I simply used a typical collimating device with a small hole in the center and
a slice of aluminum from
an old floppy disk. The two can be
permanently combined with two-sided tape.
It is important that the knife-edge be perfectly straight with no jagged
edges as seen with a strong magnifying glass. If it isn't, the accuracy of the
test will be affected. It's also important for the knife-edge to travel right
through the halfway point of the hole so as to cut right
through the center of focus.

Conducting the null test

To conduct a null test, simply place the knife-edge into
the focuser of any scope being tested.
Then while aiming at Polaris or another brighter star if the scope has
tracking, the observer will see half of the mirror illuminated while the other
appears dark. The edge or boundary
between light and dark will appear sharp or blurry. The scope should then be
placed so that the light/dark boundary goes EXACTLY through the center of the
mirror. As the focuser is adjusted, the intent
is to see the edge get increasingly blurry until the light and dark side
becomes flipped. At this point,
the focuser has gone to far and needs to be reversed just a bit until the
mirror appears equally illuminated but somewhat dimmed.

For this to work, the scope needs to be precisely aimed at
the star and the knife-edge precisely cutting through the airy disk which
requires very high pointing and adjusting accuracy to the point where it may
require patience and experience to get it right. To verify that the desired
result is being achieved, the tester can place his or her hand in front of the
scope and look for very
prominent 3 dimensional heat waves emanating from the silhouette. Once the knife-edge
is properly placed, any deviations from a perfect parabaloid should in theory
be clearly visible in
3 dimensions as brighter or darker zones.
If there is some difficulty achieving the null, try rotating the knife-edge
slightly so as to make fine adjustments as the knife-edge slowly approaches the
airy disk and allows for precise positioning.

While this test can be very revealing, it does have one
major flaw: it requires nearly perfect seeing to achieve the desired
result. Tube currents and a
turbulent atmosphere can overwhelm the shadow patterns and reduce the
sensitivity considerably.
Combining the indoor
artificial star concept and the null test

By using both the artificial star concept along with the null
test, it becomes possible to set up a genuine null test that can be used to
test a large aperture scope or smaller scopes depending on which one is KNOWN
to be of high quality. In my
particular instance, I have an 8-inch f/6 Meade Starfinder that shows a VERY
good null on Polaris. Also, when
examining the mirror with a Foucault tester and putting the numbers in Figure
XP, the mirror is found to have a .998-strehl ratio with a rating of 1/16 wave
at the wavefront and a very smooth surface.

Given that the optics are essentially perfect, I can use
this as the control scope to "look" at the mirror in my bigger or smaller
scopes. I happen to have an
18-inch dob along with a back up 17.5-inch mirror of the same focal
length. I know both mirrors have
tested in the .95-strehl range and give terrific images but now there is a way
to examine the mirrors using a null test under controlled conditions using the
8-inch. To do this I simply placed
the artificial star in the focuser of the larger scope and the knife-edge in
the focuser of the 8-inch and looking at various parts of the bigger mirror
while looking through the knife-edge.
Even though the control mirror is the smaller one it can be used to test
the larger one because the very edge of the secondary shadow and the edge of
the big mirror is visible or nearly so.
The result is that a full profile can be seen of all the zones in the
big mirror. Since both surfaces are
contributing to the null test images equally, it is actually possible to
"reverse the roles" and actually look through the focuser of the big scope with
the artificial star in the smaller scope.

Fortunately and not unexpectedly, both big mirrors I have
show a virtually perfect null and therefore I feel confident in using the big
scope as the control scope for all tests as the mirrors are more than good
enough to reveal problems in smaller scopes. Since there is some uncertainty
in what "profile " a given
optic may reveal if there is a deviation from a perfect null it may be unclear
as to which mirror that imperfection is in. All one has to do is observe the
shadow pattern observed and determine which circumference the deviation appears
to involve. For example if
one is examining a portion of a big mirror with a smaller aperture scope and
the shadow pattern appears to follow along the circumference of the big mirror
it
can be concluded that the error is in the bigger mirror. If the errors appear
to follow along the smaller mirror's circumference the error is in the smaller
scope. If a perfect null is seen it can be
concluded pretty much for certain that BOTH surfaces are good. Using this procedure,
it should be possible to see such optical errors as spherical aberration, surface
roughness,
lap marks, astigmatism, and turned edges.
If there is a question of a mirrors edge one can specifically look at
that part of the mirror and see if there is a fuzzy or darkened ring near the
edge, which would indicate a turned edge.
Since the light source can be quite dim, it is best to do this test under
darkened conditions.

It should be mentioned here that in order to do this type
of testing it is essential to have no floor vibrations or air currents in the
room. The focuser that has the
knife-edge in place needs to be adjusted to within about 1/1000 of an inch or
so of the focal point in order to work.
This again may take some time and patience to be able to achieve
consistently. The test if
set up properly will reveal sharp and contrasty heat currents from any heat
source such as a persons hand. If
heat currents are not visible when a heat source is put into the optical path,
the knife-edge is not properly positioned.
What the null test can
reveal

If an overall null cannot be achieved, then some order of
spherical aberration may be in the system. If while moving the focuser inward
the center darkens first, then there is undercorrection. If
the edge darkens first, then there is overcorrection. It is also possible that
since the entire telescope is being tested that a bad or miscollimated secondary
can affect the results. If a refractor is being tested and the
results are uncertain or questionable, the secondary should be removed so that
only the main optic is being tested.
If an achromatic refractor is being tested, it's best to place a green
filter in the optical path so as to reduce confusing chromatic effects.

One scope I recently tested for example was my Orion 80
ED. Even though images through
this scope are quite good, star testing reveals that intra and extra focal
images are not quite identical.
Using the knife-edge test with the 18-inch as the control scope, I see
that there is a slight zone appearing as a raised hill at around the 75-85 %
radius that could explain the imperfection in the star test.

Using this method can reveal other things as well. For example it is possible
to
experiment with ways to control the boundary layer above the mirror of a large
Newtonian. It is possible to
see the exact effects various fans and heating elements can have on the optical
path. Blowing air from a house fan
for example causes considerable turbulence even though the temperature is
constant because the moving air has varying densities as it moves turbulently
though the environment. The effects
of the secondary heater can also be seen as a calm rising plume of air as it
moves straight up over the secondary.

Using the null test on an
unfinished mirror

For mirror makers, it is possible to take this yet another
step further. If there is some
uncertainty in exactly what state an unfinished mirror is in, or the optician
simply wants to verify that the mirror is done, he/she can place the Foucault
tester's light source at EXACTLY the halfway point of the radius of curve,
which is of course the focal point.
By placing a high quality telescope right behind the Foucault tester,
the optician should then be able to achieve a null test with the knife-edge in
the scope's focuser but with the silhouette of the testing devise in the light
path. This in effect achieves the
ultimate test for mirror makers but without the need for expensive optical
flats or null lenses.
Concluding remarks

With some patience and experience, using this method makes
it possible to accurately evaluate a telescope's optics by using the star test
so many of us know well. In
addition, the use of the indoor null test allows observers to evaluate their
prized optics with a sensitivity and accuracy once thought to only be found in
high-end optical shops. Good luck and happy testing.